## CQSRG Online Educational Material## How to locate an earthquakeCQSRG (pronounced CQ Surge) has been researching the earthquake seismicity of Eastern Central Queensland since it began operation in 2002. |

There are several methods used to work out where an earthquake occurred. In all cases the starting point is to obtain reliable recordings of the energy waves, created by the earthquake, that travel through the ground. These seismic waves may be recorded on a number of stations at a distance from the point on the Earth's surface under which the earthquake occurred. This point on the Earth's surface is called the earthquake's *epicentre*.

Of course, the place where all earthquakes happen is not actually on the Earth's surface; it is invariably at some distance *under* the surface - usually several kilometres under the surface. The place down under the epicentre where the earthquake erruppted is called the *hypercentre* or *focus* of the earthquake.

Whilst it is relatively easy to determine an earthquake's epicentre, it can be quite difficult to determine the hypercentre.

In this article we will explain a relatively simple method used to determine an earthquake's epicentre.

When an earthquake suddenly ruptures the Earth's crust at the focus it is like a large explosion. This explosion generates a combination of *pressure* and *shear* waves in the crust that radiate outwards from the focus.

The pressure waves are *longitudinal* waves in many ways are similar to sound waves - except that sound waves travel through the air, whereas the longitudinal seismic waves travel through the Earth's crust.

The shear waves are *traverse* waves. They are somewhat similar to the waves you see when you throw a stone into a pond - except that they are not travelling over the surface. They are travelling through the body of the Earth's crust.

The compressional waves travel faster than the shear waves, and will arrive at a distant measuring station first. They are therefore called the *Primary* or P waves.

The shear waves arrive at a distant measuring station after the compressional waves. They are therefore called the *Secondary* or S waves.

It turns out that the relationship between the measured S-P time and the distance of the measurement station from the earthquake epicentre is stable enough for it to be used to calculate the approximate distance from the earthquake epicentre to the measurement station.

In Continental Crust the distance from an earthquake epicentre to a station that measures the arrival times of the P and S wave fronts can be calculated using the following approximation:

Distance(km) ~ (S-P) * 7.9

For instance, in the seismograph above, the S-P time was measured as being 14.12 seconds. Therefore the station that recorded the arrival of the waves was situated about 112 km from the earthquake's epicentre.

In order to map the distance from the measurement station to the earthquake epicentre we need to know where exactly the measurement station is located.

We see from the graphs above that the name used to designate the measurement station is **YNG**.

The International Seismological Centre (ISC) in the England maintains an online register of all official seismic monitoring stations in the World.

By searching the ISC Station Register I get the following information about the YNG station.

Code Station name/Region Latitude Longitude Elevation Depth Prime Status ============================================================================================================== YNG Young -34.29800 148.39631 460.0 Open New South Wales,Australia

This gives us the Latitude and Longitude of the station - which can be used to accurately locate the station on Google Earth Pro. Also using the features of Google earth Pro we can draw a circle of radius 112 km centred on the YNG seismic station. This indicates that the earthquake epicentre is somewhere on the circumference of that circle. Ok... now ... what if we had S-P times for another, different, monitoring station? That would allow us to draw another circle around that station - and it will intersect the circle drawn about the YNG station in (at most) two places! That would restrict the location of the epicentre to one of two possible places. If we get data from a third monitoring station we can draw a third circle, that will restrict the location of the epicentre even further to a very small area. The more stations we can get data from the more circles we can draw to confine the epicentre location more and more. |

We don't actually need the seismograms themselves from which to measure the P and S arrival times. Often various Seismology Agencies publish tables of station arrival times. The data in those tables can be used to calculate the S-P times for the various stations. We then use the ISC online register to locate the stations. Then we can use Google earth Pro to map the station to epicentre distances, and hence to locate the earthquake's epicentre.

The following table provides data from three stations that recorded the earthquake shown in the above seismograms.
The S-P times can be calculated by subtracting the P arrival time from the S arrival time. The latitudes and longitudes for each of the stations can be found by searching the online registerISC station register. We can see that the distance circles for the three stations all intersect at one place (approximately). |

The table below is a typical example of such data (note that this is data from a different earthquake to the one above).

The GA data table actually makes it very easy to map the distances - because they include their distance calculations in the table. These distances will be more accurate than those calculated using the formula presented above. Perhaps you can use the GA data to determine a more accurate factor than 7.9.

The Geoscience Australia (GA) also allows you to get an RSS feed that sends out email notifications whenever it locates an earthquake. This means that you can practice locating earthquakes using the GA published data, and also check whether your location is correct by comparing it with their map.

Have fun.